Magnetically levitated transportation vehicles have been disclosed in U.S. Pat. No. 3,768,417. These vehicles travel above a guideway trough, and are able to achieve speeds of 200 miles per hour or more.
However, in tests of scale model vehicles it was found that the moving vehicle was subject to oscillatory motion, in all five degrees of freedom, caused by irregularities in the guideway, wind gusts, nearby buildings and oncoming vehicles. Because the vehicles are inherently undamped, a perturbation tended to produce oscillations, which would create an uncomfortable ride for passengers.
A system for stabilizing the vehicles and actively damping the oscillations was disclosed in U.S. Pat. No. 3,871,301, incorporated herein by reference. In that control system, the phase of the linear propulsion field was adjusted in response to a vehicle mounted vertical acceleration sensor to generate small vertical forces to counteract instantaneous vertical acceleration. That system damped heave, or vertical motion, and also reduced the pitch oscillation, or rotation about a transverse axis, caused by heave-pitch coupling.
However, this single-axis active damping system could directly damp only vertical oscillations. It could also indirectly, but not directly, damp oscillations in all other modes, (such as pitch oscillations cited above) since all modes are coupled. That is, oscillations in any one mode produce oscillations in every other mode. This indirect damping of the other oscillation modes does not occur instantly, however, causing an uncomfortable ride.
For example, if a crosswind gust or a lateral guideway discontinuity induces a lateral oscillation (sway) of the vehicle, this swaying motion will produce vertical oscillation (heave) due to the fact that the magnetic and aerodynamic lift forces depend on the lateral position of the vehicle with respect to its guideway. This heave oscillation will be damped instantly by the active vertical damping system, and thereby the sway oscillations will also be damped, but only after they have persisted long enough to induce the heave oscillations.